14 DAY TRIAL // Canonical may be hard at work on Unity8,, but that doesn't mean that they've forgot about Unity7, especially now that it will also ship with Ubuntu 14.04 LTS (Trusty Tahr).
The default desktop environment in Ubuntu 13.10, Unity7, will also be available in Ubuntu 14.04 LTS (Trusty Tahr), but with a few upgrades.
Until now, the themes have been using the metacity engine, but Canonical has decided that in order to upgrade Unity, developers must also upgrade the codebase.
“Until 13.10 unity supported the metacity theming engine, through the compiz native gtk-window-decorator, however this system is quite old right now, and has never been ported to gtk3 to get better theming capabilities and support gtk-css theming.”
“So, as part of the cleanup for Ubuntu 14.04, we defined a completely new theming system based on GtkStyleContext, that uses CSS for pretty much everything,” reads the official wiki.
The developer in charge of upgrading Unity7 has shown us some of the progress made and it looks great. The downside of this upgrade is that the current themes won't work, unless they are upgrade by their respective designers.
This is definitely worth it, because full Gtk3 Theming is now supported and the windows will feature anti-aliased corners.